Environmental Engineering Reference
In-Depth Information
Boron
0.75
2.0
0.75
2.0
Cadmium
0.01
0.05
0.01
0.05
Chromium
0.1
20.0
0.1
1.0
Copper
0.2
5.0
0.2
5.0
Nickel
0.2
2.0
0.01
0.05
Lead
5.0
20.0
5.0
20
Zinc
-
-
2,0
10.0
Manganese
-
-
2.5
2.5
After: * - WWEDR 2000
** - Papadopoulus (1985)
Cadmium is very mobile and is readily available to crops. It accumulates in crops without
showing signs of phytotoxicity.
Pb exists in three major oxidation states (Pb
4+
, Pb
2+
, Pb
0
). Solubility of Pb compounds
is strongly dependent on pH and redox potential. The main sources of Pb in wastewater
are batteries, cable sheathing, paints, and plastics. Uptake of Pb by plants is low even
under conditions of a high Pb in soil solution and most of it is retained in the roots (Wild
1995).
Sources of Zn in wastewater include metal coating, alloys, batteries and pigment. A
high Zn content in soil solution can be phytotoxic and reduces growth (Landon 1991,
Henning et al. 2001). The remedy to Zn toxicity is to lime the soil to pH 6.5. Zn is most
readily absorbed than any other trace metal and has the potential to move below the depth
of incorporation (Oloya & Tagwira 1996, Rechgil 1995, Pol-prasert 1989). Crops
sensitive to Zn deficiency include maize, citrus, alfalfa and cotton. Soil parent material,
organic matter content and soil pH affect Zn availability. Zn deficiency is associated with
a high pH, coarse-textured and highly leached soils and soils with low inherent zinc
concentration (Mikkelsen & Camberato 1995).
In soils, Cu exists mainly as Cu
2+
adsorbed to clay minerals or organic matter. Crops
sensitive to copper deficiency isnclude cereals and vegetables. Copper toxicity is not very
widespread but concentrations greater than 150 ppm are toxic to citrus (Landon 1991).
Like other trace metals, solubility of copper is highest at pH less than 5.
4 IMPACTS ON SOILS
4.1
The study area
The study was carried out at Imbwa Farm in Beatrice with a total area of 1444 ha of
which about 481 ha were under irrigation. Before pasture irrigation, the farm was used
for the production of tobacco under dry land conditions. Since 1979, the farm has been
converted to wastewater-irrigated pasture, which is the most common wastewater-
irrigated crop in Zimbabwe. Grass species grown on the farm include a mixture of
